This week’s Growback doesn’t have any great ecological message, but the patterns are fun and deserve a blog.
How are your API (air photo interpretation) skills? API is becoming an archaic art form nowadays. Before it dies out, here’s a visual quiz to see how well you can interpret vegetation patterns from the air. First the easy questions. How many animals can you see in the photo above? Can you find the dog and the dolphin? What else can you see?
Now the ecology questions. Why do the trees grow in rings? What kind of trees are they? What happened, about 70 years ago, to form the curious shapes?
If you’re puzzled, the clues lie in the brown areas surrounding the tree rings. You can click on the photo to enlarge it and see more details. The next photo shows the same area about 70 years earlier. The shapes are much the same, but the vegetation is very different. Here’s three more questions to test your API skills: what are the big white blobs? Why aren’t any trees growing on or around them? What caused this pattern to change to the one above?
The photo below provides the missing link. Did your explanations ‘hold water’?
The photos show bands of river red gum (Eucalyptus camaldulensis) forest on the Murray River at Lake Mulwala. The black and white photo was taken in 1941, just before the lake was dammed. The photo of the flooded lake was taken in 2005, and the top ‘animal shape’ photo was taken in 2009, when the lake was emptied to control the invasive aquatic weed, Egeria (=Elodea) densa (Leafy Elodia).
It’s easier to grasp the ecological context of the ‘animal shapes’ and the ‘big white blobs’ from a landscape view. The sinuous river in the black and water photo below is the Murray River.
In the photo below, the channel of the river can just be seen beneath the flooded lake.
And here are the animal shapes again in 2009…
I assume you worked it all out. The animal shapes are islands, fringed by bands of river red gums, which grow at the water’s edge. Before the lake was built, the floodplain supported a large red gum forest. Nowadays the lake bed is covered with fallen trees and dead stags. Don’t ask me why, but lots of people love gazing at flooded, dead tree trunks so the lake shore is prime real estate.
Two questions remain: what are the big white blobs on the 1941 photos, and why don’t trees grow across the entire islands rather than just around the edges?
The white areas are sand hills, which formed in an earlier, drier epoch. The sandy rises are too dry to support red gums, which is why red gums grow around the edges of the islands but haven’t colonized the island tops, even in the flooded lake.
Originally, the bare sand hills may have supported Murray Pine (Callitris gracilis ssp. murrayensis) or White Cypress-pine (Callitris glaucophylla). Callitris were widely chopped down in the 1800s for timber and to clear the sandy soils for cropping. Nowadays, most sand hills are extremely degraded and covered with exotic annual grasses and herbs such as Paterson’s Curse (Echium plantagineum). Consequently, Sandhill Pine Woodland is now an endangered community in NSW.
To compensate in a very minor way for the many hectares of flood-killed forest, red gums have expanded on the floodplains upstream of the lake. Perhaps this was triggered by changes to flooding patterns after the lake was built. The following photos show a stretch immediately upstream of Lake Mulwala, where river red gums have colonized areas that were once cleared pastures.
So there you have it. Sandhills + Water = Animal Crackers. Just add water.